1. Field of Invention
The present invention is related to hydro-electro-winning or electro-refining of metals, such as copper, zinc and nickel, particularly to a cathode plate which is a substrate for precipitating metals thereon in the form of a sheet during the hydro-electro-winning or electro-refining. The above-mentioned cathode plate and precipitated metal are referred to as the mother plate and the mold plate, respectively. The present invention is more particularly related to the mother plate having an improved insulating structure for shielding its edge portion from the electrolytic precipitation of metals.
2. Description of Related Art
The edge portion and side of a mother plate have been coated with an insulator so as to enable the mold plate to be peeled from the mother plate. Since the electrolyte liquid for obtaining the mold plate is based on sulfuric acid and is acidic, the insulator used as the coating of the mother plate's edge portion must be resistant to the sulfuric acid. The insulator must also be well bonded to the mother plate.
The conventional coating methods of the insulator are provided for example (a) bonding a vinyl-chloride sheet, (b) applying an insulating resin, (c) applying a heat-resistant resin such as epoxy resin via a vinyl-chloride sheet (Japanese Unexamined Patent Publication No. 50-160,121), (d) applying a polyurethane rubber (Japanese Unexamined Patent Publication No. 59-13,089), (e) applying a fluorine rubber (Japanese Unexamined Patent Publication No. 63-307,292), and (f) applying a resin which is mainly composed of dimethyl polysiloxane (Japanese Unexamined Patent Publication No. 2-61,086).
The method (a), in which a vinyl-chloride sheet is applied, incurs problems such as: the adhesion of the sheet decreases during the electrolyzing; the electrolyte liquid penetrates between the mother plate and the insulator; and, the metal inevitably precipitates in the clearance between the mother plate and the insulator during the electrolyzing. When the mold plate is peeled from the mother plate, the metal, which has been precipitated onto the edge of the mother plate, is incorporated is the vinyl chloride. Alternatively, the precipitated metal so grows to such an extent that the vinyl-chloride sheet ruptures. The method (a) is, therefore, disadvantageous in the point that the life of the insulator, i.e., the vinyl-chloride sheet, is very short.
The method (d) can eliminate these advantages since the dimethyl polysiloxane used has good flexibility and sealing property.
The method (c) eliminates the high-cost problem of the method (a). The method (d) emphasizes the excellent acid-resistance of polyurethane rubber and aims to improve the shielding performance of the insulator against the electrolytic precipitation.
In addition, in the conventional insulator-coating methods (c) through (f) the coating is provided on the mother plate at its peripheral or edge portions of the main surface.
The conventional coating materials, which are applied as the insulator of the peripheral or edge portions of the mother plate, are resin and rubber. These materials are usually referred to as the protector. The protector is applied by means of application with a brush or bonding with a binder. However, since the bonding property of the resin materials of the protector with the metal of the mother plate is poor, the insulator function is lost in a relatively limited number of times of the electrolysis process and hence must be replaced. In addition, the mother plate has to be polished after lapse of a certain period. Since the conventional material is applied on the peripheral or edge portions of the main surface of the mother plates, such material must be preliminarily removed when the polishing is carried out.